The bacteria that live in the human gut may play an important role in immune response to vaccines and infection by wild-type enteric organisms, according to two recent studies resulting from a collaborative effort between the University of Maryland School of Medicine Institute for Genome Sciences and the Center for Vaccine Development. The first study, published online in PLOS ONE, examines the impact of an oral typhoid vaccination on the microbiota, or populations of bacteria, in the human gut. The second study, also published online in PLOS ONE looks in monkeys at the impact in the gut microbiota of vaccines against Shigella, as well as exposure to wild-type Shigella, another group of bacteria that, like S. Typhi, gain access to the host via the oral route. These studies find that higher diversity in the gut microbiota, i.e., more types of bacteria in the gut, affect the characteristics and magnitude of the immune responses to the vaccines and, in the case of exposure to wild-type Shigella, appear to be more resistant to infection. This research provides a window into how vaccines and resistance to enteric pathogens work. It also helps scientists understand more about how the "good" bacteria in the body affect human health, a growing area of research known as the human microbiome.